Landscapers typically select drought-tolerant vegetation when designing rock gardens, but in regions where deer are a problem, your rock garden should also contain deer-resistant, or deer-tolerant, plants. These stalwarts handle both drought and Bambi pretty well, and they can be grown most anywhere in the continental U.S. (and across corresponding temperate zones). And because they are going into a rock garden, they not only thrive in dry (but well-drained) soil, they also have a preference for sunny conditions, since rock gardens tend to have lots of sun but little water.
Hens and Chicks
Hens and chicks (Sempervivum tectorum) is a deer-resistant plant that forms attractive rosettes. Its succulent leaves mass together into short, compact mounds. Hens and chicks does bloom, but the plant is grown for its foliage, not for this negligible, spiky flower. The tiny "chick" plants grow at the base of the "hen," or main plant. If you want to propagate, you can detach the chicks and grow them elsewhere. Otherwise, just let them be, and they will form a dense mat that essentially serves as a ground cover.
Sedum and Other Stonecrops
Stonecrop plants are a perennial favorite in rock gardens, as the "stone" in the name might suggest. Common examples include sedums, such as Angelina sedum and Autumn Joy sedum. Stonecrop foliage consists of succulent leaves in whorls. The leaves are sometimes variegated and can range in color from bluish-green or greenish-yellow to reddish-pink or almost off-white. Unlike hens and chicks, stonecrop produces a flower well worth growing in its own right. Stonecrop flowers can be yellow, orange, red, pink, or white. The flowers usually bloom in clusters above the foliage.
Prickly Pear Cactus
Barbed-wire fences may be effective in controlling deer—if the fences are tall enough. But why not put the barbed-wire right on the plant you need to protect? This is just what prickly pear cactus does. Prickly pear cactus (Opuntia compressa) grows to be about 6 to 14 inches tall. It bears showy yellow flowers, 2 to 3 inches in diameter, as well as those menacing spines. A prickly pear cactus in bloom positioned next to a red hen and chicks plant makes for a striking rock garden pairing. This is the only cactus found widely in the eastern United States.
Your choices for deer-tolerant plants in rock gardens are not restricted to cacti and succulents. Lamb's ear (Stachys byzantina) provides wonderful texture in rock gardens and spreads readily. Lamb's ear plants produce light purple flowers on tall spikes. Their silvery foliage has a velvety texture, which is how lamb's ear got its name. Apparently, this same texture is unpalatable to Bambi and makes lamb's ear a deer-tolerant plant.
Purple Wood Spurge
There are all kinds of reasons why deer may decide not to eat a particular plant. Besides off-putting textures and the presence of spines, toxicity can be a deterrent. Such is the case with purple wood spurge. Of course, it is not ideal having poisonous plants in the landscape if young children will be playing there (or if you have pets that go outside). Another drawback with purple wood spurge is that it tends to spread to areas of the yard where you do not want it to grow. But if neither of those drawbacks is an insurmountable obstacle for you, purple wood spurge may be worth a look. It certainly is an interesting one to observe growing from season to season.